1. Field of the Invention
The present invention relates to a camera and specifically to improvements in a camera having an automatic focus detection device.
2. Description of the Prior Art
Various types of automatic focus detection devices or automatic focus adjusting devices for a camera have been proposed heretofore and some of them have been embodied in commercial products being built in cameras sold in the market.
However, when these various devices including those being used in commercially sold products are carefully examined, it is revealed that a focus detecting accuracy of such devices is not always good, and there are many instances where accurate focus detection can not be done depending on photographing conditions.
For example, there is a device of passive type employing one or more photoelectric elements among said various types of devices, but in such a device when the object brightness lowers or the contrast pattern of an object is complicated accurate focusing detection can no longer be expected.
Also there is an active type device which has a light beam projector to project light beam onto an object and one or more photoelectric elements to receive the light beam reflected from the object, but light reflecting planes of an ordinary object are rough and unsmooth in most cases, further the intensity of light reaching a certain point is inversely proportional to a square of a distance thereto, therefore it is very difficult to always secure sufficient intensity of the reflected light beam impinging into the photoelectric elements in said device, thus such cases are apt to take place that accurate focus detection can not be achieved.
Therefore, what is desired when such an automatic focus detection device or automatic focus adjusting device is actually built in a camera is to provide some form of an auxiliary device which can take place of said automatic device when the automatic focus detection function thereby is not satisfactorily done by said reasons, as a means to relieve the situation so that a practically satisfactory photographing can be done automatically.
That is, unless such auxiliary device is provided, a user of such camera having said automatic focus detection or automatic focus adjusting device built therein will have no choice but is required to make such troublesome handling as judging a distance to the object with his eyes and then making the focus adjustment using a distance scale on a photo-taking lens when said automatic device does not function effectively, thus advantages of said automatic focusing detection device are totally lost, further such automatic function constitutes a difficulty in photo-taking in some cases.
In view of the above, such camera has been proposed as having an automatic focus adjusting device and further having a coordinating means to coordinate focus adjustment with exposure aperture adjustment.
That is, a camera proposed by U.S. Pat. No. 3,618,499 (corresponding German DT-OS No. 20 15 899) is a one, in which a photo-taking lens is so made as being shifted continuously by an urging force of a spring member from its nearest photographing distance position (for example a position corresponding to 1.2 meters) toward its infinite photographing distance position, and at the same time an automatic focus adjusting device of an active range finding type is so provided as initiating its action to detect an object distance in response to said shifting of the taking lens and latching the taking lens when the taking lens reaches such photographing distance position as corresponding to the object distance in a course of said shifting. Further in this camera, an exposure meter to respond to the output of a light measuring photoelectric element, and an aperture adjusting means to adjust a diaphragm means according to an aperture value (f-stop value) indicated by said exposure meter are provided, and a means is provided at a part of said aperture adjusting means to limit an adjustable range of focus of said taking lens according to the exposure aperture value, wherein the adjustable range of focus of the taking range is adjusted stepwise according to the aperture value by an action of said limiting means in such manner as for example at a range of 1.2 meters to infinite in case of exposure aperture value of f3.2 (which means the maximum aperture size of the camera), also at a range of 1.2 meters to 7.2 meters in case of f5.6, and at a range of 1.2 meters to 3.6 meters in case of f11 (which means the minimum aperture size of the camera). Here, the figures shown above are approximate data when the focal length of a photo-taking lens is 44 mm.
Therefore, in this type of camera, the effective focus adjusting range of an automatic focus adjusting device is limited in such a manner as at a range of 1.2 meters to infinite in case of f3.2, and at a range of 1.2 meters to 7.2 meters in case of f5.6, while at a range of 1.2 meters to 3.6 meters in case of f11, according to an exposure aperture value, and in an event said focus adjusting device fails to detect an object distance within said ranges in actual photographing, the taking lens wil be automatically set according to an exposure aperture value for example at a position corresponding to an infinite distance in case of f3.2, and at a position corresponding to a distance of 7.2 meters in case of f5.6, while at a position to a distance of 3.6 meters in case of f11.
That is, such camera takes an advantage of characteristics of the so-called hyper focal range of a lens that is all objects within a range from a certain limited distance to the infinite distance can be focused with a permissible degree of sharpness, wherein for example when the exposure aperture value is f3.2 in a lens having a focal length of 44 mm, the hyper focal distance (a photographing distance to which the lens is to be match for obtaining the above mentioned hyper focal range) is about 13.5 meters (which is already equivalent to infinite in case of ordinary phototaking lens) wherein the hyper focal range at this time is about 6.75 meters to infinite, and in case of f5.6, the hyper focal distance is 7.2 meters and the hyper focal range at that time is 3.6 meters to infinite, further the hyper focal distance in case of f11, is 3.6 meters and the hyper focal range at that time is 1.8 meters to infinite. That is, in this camera, when the automatic focus adjusting device fails to detect the object distance within each effective focus adjusting range mentioned above, for example if the exposure aperture value is f3.2, the taking lens is set at a position corresponding to infinite, and then objects within a range of 6.75 meters to infinite can be focused with permissible degree of sharpness, and if it is f5.6, the taking lens is set at a position corresponding to 7.2 meters, and then objects within a range of 3.6 meters to infinite can be focused with permissible degree of sharpness, further, if it is f11, the taking lens is set at a position corresponding to 3.6 meters, then objects within a range of 1.8 meters to infinite can be focused with permissible degree of sharpness.
As has been explained above, a camera proposed above has a means of relief for a case when the automatic focus adjusting device can not effectively function, by utilizing a combination of the so-called hyper focal range of a lens and an exposure aperture size thereof, but the system mentioned above in this camera is apparently unreasonable in the following points.
That is, since the adjustable range of focus of a lens is singularly limited according to the exposure aperture value, useful properties specifically provided at the automatic focus adjusting device are left alone in vein, especially under such a photographing condition that the exposure aperture size becomes comparatively smaller, and the significance for providing the automatic focus adjusting device is more or less halved.
For example, even when an object is at a position of 5 meters further the automatic focus adjusting device is in a state being able to sufficiently detect said object distance, if the exposure aperture value is indicated at f11 by the exposure meter, the taking lens will be set at a position corresponding to 3.6 meters by the action of said limiting means even if the automatic focus adjusting device is in such state as being able to detect the object distance with sufficient accuracy, thus the significance of providing said automatic focus adjusting device is totally lost.
And in this case while the hyper focal range of 1.8 meters to infinite can be obtained by setting the taking lens at the position corresponding to 3.6 meters, said hyper focal range is nothing but a range in which quasi focusing can be obtained, therefore even if the hyper focal range of 1.8 meters to infinite can be obtained by setting the taking lens at the position corresponding to 3.6 meters in case the exposure aperture value is f11, only an object at the position of 3.6 meters can be precisely focused but objects at other positions are out of focus in an exact sense of words.
Therefore, it is important in this kind of camera to provide such design that the above mentioned limiting means that is a coordinating means for coordinating the focus and exposure aperture adjustments is utilized only as an auxiliary relief means in case the automatic focus detecting or adjusting device can not effectively function, that is such design that the effective focus detecting or adjusting range of the automatic focus detecting or adjusting device is always secured within a range from the minimum distance allowing a photographing to an infinite distance in order to utilize the function of said device to its maximum extent in an ordinary case, and only in case when the automatic focus detecting or adjusting device can not effectively function in photographing, said coordinating means can be utilized as the relief means therefor.